This is the only airdrop of merit that the forum have carried out, the rest are giveaways carried out individually by forum users.
For current members, your initial merit score is equal to the minimum required to your rank. Of that, a certain amount (less than the usual half) is spendable. The spendable amount was calculated based on your current rank and the number of activity points you earned in the last year. A Legendary member who hasn't posted in the last year would still be Legendary, but would not have any spendable merit.
Member rank users got 10merits
Full memeber rank users got 100merits
Snr member rank users got 250merits
Hero member rank users got 500merits etc
Before the merit system was implemented, only activities were required to rank up so the airdrop was done to correspond with users current rank.